I was looting up my character, after just spawning in at the coast, and I tried picking up the first backpack I found. This backpack was half inside the wall of a double storie green house. I assumed that backpack was left there by another player or someone took it, but server-side the backpack was still on the floor for me. I tried dragging it into my character, into the the small square at the top right of inventory and picking it up in my hands, all in this order. Nothing worked.

I moved on looking for another backpack that I could pick up and I found a mountain backpack on top of a tower. I tried picking it up but I could only put it on my hands. After this every single item that I would take would just dissapear. I soon realised that first backpack I've found was invisible in my back, so I left the server and joined back in and... there it was, the first backpack I found was in my back along with all the items I took (by double cliking or pressing "take")